Why might they not see it as in their best interest to ally themselves with Great Britain

This is because the French were not waging war with the Colonials, they were waging war with England. Hawkeye, and the Mohicans knew this, and had no interest in fighting the French. Low and behold, the story progressed and they ended up fighting with the British, not to save British lives, but to save the lives of the helpless. This can be witnessed during the George Road Massacre, and the Evacuation of Fort William Henry Massacre.